The present invention relates to a germanium detector for measuring an energy spectrum of a gamma ray flux, and to a method for measuring a photonuclear reaction cross section using such germanium detector. The invention also relates to a method for determining a peak value of a photonuclear reaction cross section by using the same method in transforming an atomic nucleus.
Dealing with a high level radioactive waste from an operating nuclear reactor is a serious problem. One general method is the entombment of high level radioactive waste in a vitrified or asphaltified mass. The mass is kept until the radioactive waste decays to eliminate the radioactivity. This type of method can deal with radioactive waste including cesium 137 and strontium 90 of a relatively long half-life. However, the mass should be maintained somewhere over a long period of time. It is very difficult to find and maintain a site for storing the mass. Accordingly, other than the natural decay of radioactive waste, a treatment is sought for promoting the decay of a radioactive waste.
One proposed treatment comprises neutron irradiation in which radioactive waste is irradiated with neutrons. This treatment utilizes (n, .gamma.) reaction or/and (n, 2n) reaction triggered by neutrons. Such a reaction serves to transform a nuclide to be dealt with into a short half-life or stable nuclide. However, an apparatus has not yet been developed for generating the high density neutron flux required for such a reaction. Therefore, the ability to provide such treatment is still doubtful until a practical nuclear fusion reactor can be produced in the future. Thus treatment without a neutron flux should be considered at the present time.
For example, Japanese Patent Publication (kokoku) No. Hei 1-51158 (1989) discloses a treatment which meets the above requirement. The disclosed treatment uses a white gamma ray flux in the energy range of 10 MeV-25 MeV with which a radioactive waste containing subject nuclides is irradiated. The irradiation serves to trigger photonuclear reaction such as (.gamma., n) reaction so as to promote the transformation of the subject nuclides. FIG. 1 shows that an electron beam 6 is generated in an electron accelerator 10 in this treatment. A convertor 2 converts the electron beam 6 to gamma rays 17, with which a subject nuclide 4 is irradiated. The irradiation transforms the subject nuclide 4.
The treatment was established in accordance with the following phenomenon. Generally, the irradiation of a subject nuclide with gamma rays of at least 7 MeV-8 MeV enables photonuclear reaction. When the energy of the gamma rays reaches 10 MeV-25 MeV, the subject nuclides abruptly come to absorb the gamma rays quickly, so that a resonance phenomenon of several hundreds millibarns takes place in this energy range, which is called "giant resonance reaction (absorption)". The photonuclear reaction is frequently generated in the giant resonance reaction. This promotes the transformation of the subject nuclides, for example, from cesium 137 to cesium 136. Cesium 137 decays with the emission of a beta particle by the radioactive half-life of 30 years, while cesium 136 decays by the radioactive half-life of 13 days to be barium 136, a stable nuclide. This is the summary of the treatment.
However, the short half-life of 13 days is ideally achieved only when all nuclei of cesium 137 are transformed to those of cesium 136. Therefore, the average half-life is actually longer than 13 days (but shorter than 30 years).
The prior art treatment will be further described in detail using the concept of a photonuclear reaction cross section (unit: "barn"). A photonuclear reaction cross section is nearly proportional to a probability of the occurrence of photonuclear reaction.
In the prior art treatment utilizing photonuclear reaction, radioactive waste is irradiated with a white gamma ray flux including an energy level which has a maximum photonuclear reaction cross section (referred to as an "optimum energy level" hereinafter). It is definitely preferable to use a monochromatic gamma ray flux concentrated at an optimum energy level in place of a white gamma ray flux, which includes a broad band of energy level. The monochromatic gamma ray flux enables photonuclear reaction to take place more efficiently. However, monochromatic gamma rays have not been used in the prior art treatment because a photonuclear reaction cross section cannot be measured accurately enough.
In a prior art method for measuring a photonuclear reaction cross section, a subject nuclide is irradiated with a monochromatic gamma ray flux, thereby triggering photonuclear reaction in the nuclide. The number of neutrons generated by the photonuclear reaction is then counted using a neutron detector. Varying the energy level of the applied monochromatic gamma rays reveals the photonuclear reaction cross section of the subject nuclide at the varied energy levels. The prior art method provides, as shown in FIG. 2, an energy spectrum of photonuclear reaction cross section of cesium, which indicates the optimum energy level around 15 MeV. The prior art method depends on the accuracy of energy level in generating a monochromatic gamma ray flux, resulting in a 3% resolution of measurement of a photonuclear reaction cross section.
The 3% resolution of measurement provides a question whether or not an energy spectrum in FIG. 2 obtained from a neutron detector reflects a precise photonuclear reaction cross section. Actually, it is possibly suggested so far that some nuclides have a peak for a photonuclear reaction cross section at a specific energy level. If a photonuclear reaction cross section is supposed to have a keen peak at a certain energy level, so that a slight deviation from the peak causes a drastic decrease in the photonuclear reaction cross section, it is not desirable to determine a photonuclear reaction cross section using the spectrum shown in FIG. 2. Further, the generation of a monochromatic gamma ray flux, for instance, by the collision of an electron and a positive electron, costs much more as compared with that of a white gamma ray flux. The practical utilization of a monochromatic gamma ray flux may therefore fail to cover the investment, because it is uncertain how much probability can be achieved to trigger photonuclear reaction with a monochromatic gamma ray flux having an energy level supposed to be an optimum energy level.
Moreover, an energy spectrum in FIG. 2 of a photonuclear reaction cross section allows some modification due to a 3% resolution of measurement by moderating up-and-downs in the spectrum of the photonuclear reaction cross section. The spectrum thereby is expressed by a smooth curve. The maximum value observed in FIG. 2 may be plotted much smaller than the actual value.
The calculation is made to reveal the amount of energy that is consumed in a prior art treatment. For this purpose the energy required to transform the nuclide of cesium 137 in a prior art treatment is compared with the energy required to generate cesium 137. In a prior art treatment, a photonuclear reaction (.gamma., n) causes an electron pair creating reaction in the competitive process. It is known that the photonuclear reaction cross section .sigma.1 of cesium in FIG. 2 is equal to 0.32 barn at 15 MeV whereas the reaction cross section .sigma.2 of an electron pair creating reaction is approximately equal to 9 barns. Suppose that the ratio .sigma.1:.sigma.2 can be approximated to 1:29. This means that one photon of gamma rays of 15 MeV is spent to generate a photonuclear reaction while 29 photons are consumed to create electron pairs. The total energy of 450 MeV corresponding to 30 photons of the gamma rays causes a photonuclear reaction so as to transform a nuclide of cesium 137. The total energy is merely estimated when a monochromatic gamma ray flux is radiated at the optimum energy level. The Monte Carlo Simulation reveals that a total energy of approximately 3000 MeV is required for a white gamma ray flux in the prior art treatment. It is well known that 0.06 nucleus of cesium 137 is formed at one fission which consumes approximately 200 MeV. In other words, a heat energy of approximately 3000 MeV (=200 MeV/0.06) can be obtained for every generation of a cesium 137 nucleus. The heat energy can be converted to electricity at an efficiency of 1/3. The final energy thus obtained from the generation of a cesium 137 is estimated to be approximately 1000 MeV. Accordingly, the prior art treatment of cesium 137 cannot stand against a cost performance requirement because the energy of 3000 MeV is required to eliminate a nucleus of cesium 137 which provides only the energy of 1000 MeV.